1. Field of the Invention
The present invention relates generally to electrophotographic (EP) imaging machines and, more particularly, to a system for tailoring a transfer nip electric field for enhanced toner transfer in diverse environments.
2. Description of the Related Art
Color EP imaging machines, such as color laser printers, typically utilize an intermediate transfer belt to accumulate a final output image from a plurality of individual images, known as separations or layers. The layers are placed onto the intermediate transfer belt in succession as the belt passes by a photoconductive (PC) drum associated with each of the different color, first transfer, stations. Once the intermediate transfer belt has traversed all of the PC drums the resulting, or final, output image will be transferred to a print medium, for instance a sheet of paper, at a second transfer station. The system of this color laser printer is known as a two transfer system.
Diverse environments create difficult situations for transferring toner in color laser printers. In cold/dry environments the media material and transfer components are highly resistive and it takes longer to build a transfer electric field. In hot/wet environments the media material and transfer components are very conductive and do not perform well the capacitive function needed to build a good transfer electric field.
In the two transfer system, toner is collected on the intermediate transfer belt after passing through the multiple, successive, first transfer stations. As toner passes through each of the successive stations, it gains charge from the post-nip breakdown which happens between the non-toned regions of the photoconductor (PC) drum, which have higher charge, and the belt. For this reason, toner placed on the belt in an upstream station gains more charge than toner placed on the belt in a downstream station. The inequality of the charge entering the second transfer nip contributes to difficulties in properly transferring both single layer, low charge toner and multi-layer, higher charged toner to the final media.
If the voltage or current range over which good transfer can occur (transfer window) is relatively large, then this difference in toner charge is not significant to transfer performance. The voltage or current is simply increased to the point where all toner can be successfully transferred. If, on the other hand, at the second transfer system there is difficulty creating a good electric charge field, a multitude of defects can result which cannot be adequately compensated for by simply increasing the voltage or current.
The most common defect caused by this problem is a washing out of the lowest charge single layer toner, normally the black toner, due to Paschen breakdown. The most common solution to this problem is to put other toner layers under the black to artificially both darken it due to the additional toner and modify the electric field at which it will transfer correctly because the added toner is higher in charge. While this solution is effective in creating good quality prints in difficult environments, it has some significant disadvantages. The most significant disadvantage from a print quality point of view is that it does not address other occurrences of poor transfer caused by the extreme environment that shows up in the other colors.
The under-laid toner also reduces color cartridge yield, the number of printed sheets a cartridge can be expected to deliver under normal printing. Under-laying black toner also requires very good registration and color linearization as well as requiring color printing at all times which can increase wear on the whole printer. While under-laying black toner with process black is a good solution to get very high quality prints in certain circumstances, it is not the best option to employ at high temperature and humidity.
Several mechanisms are at work creating transfer problems in hot wet environments. The first of these is that sheets of paper have a variety of moisture acclimation levels. Very saturated paper is extremely conductive and can conduct current laterally within the paper itself. Lateral conduction of current can be a problem both for two transfer and single transfer systems when the current flow is significant as compared to the current required to transfer toner to the print medium.
When paper conducts current in the process direction it can cause loss of electric field by draining current to other components at other potentials (e.g. at ground potential) and it can cause non-uniform, pattern-dependent transfer. The circuit model of FIG. 1 demonstrates how this can happen. If the resistivity of the paper, represented by the resistors Rp, Rprocess and Rlateral is large, then current will travel down through the two parallel stacks of toner without much regard for the resistance and charge encountered in the toner. Very little current will go off to the sides because side paths are higher in resistance. However, if paper resistance is smaller, then the current will divide and some will cross over to go down the path of less resistance. This would means that lower charged thinner layers of toner would receive more current and thicker, higher charge layers of toner would receive less. The result of this is to decrease the voltage/current at which the thinner, lower charge layers come into and go out of the transfer window, and increase the voltage/current at which thicker, higher charged layer come into and go out of the transfer window. In situations where overlap of these two windows was already difficult, this aggravates the problem.
For transfer systems at hot/wet environments, more conductive paper also means increased charge migration from the transfer member side of the paper to the toner side of the paper. Charge on the surface of the paper can either initiate Paschen Breakdown (a voltage at which the insulation of air breaks down and an avalanche condition ensues allowing flow of ions) or, just as likely, discharge toner trying to transfer. Either occurrence produces areas of poor transfer efficiency because of the neutral and wrong sign toner created at the nip entrance. Solutions to address this problem have the undesirable result of hurting performance in cold/dry environments. In cold/dry environments rolls and paper require long nip time and large nips to enable formation of good transfer electric fields. In hot/wet environments where everything is more conductive, large nips increase current migration which leads to single-layer toner wash out.
Extreme current migration can also lead to non-uniform transfer of half tones and solids giving a mottled or “crunchy” look. A mottled toner defect caused by this problem will be referred to as “crunchy” defect. A transfer geometry that brings nips together as electric fields build up can reduce current migration, but low resistivity components allow the system to more rapidly go into pre-nip over-transfer, thus creating small transfer windows. In cold and dry environments, these types of nip geometries make building large charge fields difficult without pre-nip Paschen Breakdown.
Thus, there is still a need for an innovation that will deal satisfactorily with inequality of the charged layers of toner entering a transfer nip charge field in diverse environments.